Electric door strikes are commonly used in various places of business where it is desired to control entry into a secured area by means of a remote switch. As an example, the lobby of a building might be separated from the rest of the facility by a door that is secured by an electric door strike. When an individual or group of individuals has been cleared for entry into the main part of the building, the receptionist or security guard releases the strike by means of the switch.
There are two general versions or operating modes of the electric door strike, commonly referenced as "Fail Secure" and "Fail Safe".
In the Fail Secure mode, a loss of power leaves the door strike in the locked condition while in the Fail Safe mode, a loss of power leaves the door strike in the unlocked condition.
A very desirable feature of an electric door strike from the installer's point of view is a simple and inexpensive means for converting a given strike from Fail Secure to Fail Safe or vice versa. It is also desirable to be able to accommodate a wide range of supply voltages so that a given door strike can be controlled by either 12 or 24 volts, for example.
For both operating modes it is also highly desirable to minimize power dissipation in the electric door strike. Low dissipation is important because the strike must operate in a very limited and confined space under less than ideal cooling conditions. To accommodate this requirement, the present invention is directed toward the use of a latching solenoid as an element of an electric door strike. The latching solenoid is set to an extended plunger condition or state by a current pulse of a first polarity and it is set to a withdrawn plunger condition or state by a current pulse of the opposite polarity. The set plunger condition is sustained between current pulses without the benefit of a holding current. Such low duty cycle pulsed operation translates into low power dissipation.
Magnetic latching solenoids are available in several varieties, the most common type being the push solenoid which latches magnetically in the plunger extended condition. When released from this condition by a pulse of the appropriate polarity, the plunger is driven to the withdrawn condition by a spring. The extended condition is thus sustained magnetically and the withdrawn condition is sustained by the spring, both without the benefit of holding current between current pulses.
Another class of latching solenoid is a pull type wherein the magnetically latched position is with the plunger retracted and a spring forces the plunger to the extended state. Of course, configurations of the solenoid also are available where the spring is replaced by yet another permanent magnet and both plunger conditions are sustained magnetically.
Recently, a pull type solenoid was introduced which has a permanent magnet so located as to hold the plunger in the extended position. This version is particularly useful in powering electric strikes because it allows the full force of the pulsed solenoid to withdraw the plunger while at the same time using a very strong permanent magnet to ensure that any vibration or intentional pounding on the end of the solenoid does not start the plunger into false withdrawal motion. This solenoid is also available as a double latching solenoid, i. e. with magnetic latches for both positions.
The present invention comprises a simple control circuit for a latching solenoid. The control circuit of the invention together with the latching solenoid are intended to be incorporated in an electric door strike exhibiting lower power dissipation and reliable fail safe or fail secure operation.